1. Field of the Invention
The present invention relates to a semiconductor laser device having a current confinement structure and an index-guided structure. The present invention also relates to a process for producing a semiconductor laser device having a current confinement structure and an index-guided structure. Further, the present invention relates to a solid-state laser apparatus which includes as an excitation light source a semiconductor laser device having a current confinement structure and an index-guided structure. The solid-state laser apparatus may include provision for generating a second harmonic.
2. Description of the Related Art
(1) In many conventional current semiconductor laser devices which emit light in the 0.9 to 1.1 xcexcm band, a current confinement structure and an index-guided structure are provided in crystal layers which constitute the semiconductor laser devices so that each semiconductor laser device oscillates in a fundamental transverse mode. For example, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.102 discloses a semiconductor laser device which emits light in the 0.98 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al0.48Ga0.52As lower cladding layer, an undoped Al0.2Ga0.8As optical waveguide layer, an Al0.2Ga0.8As/In0.2Ga0.8As double quantum well active layer, an undoped Al0.2Ga0.8AS optical waveguide layer, a p-type AlGaAs first upper cladding layer, a p-type Al0.67Ga0.33As etching stop layer, a p-type Al0.48Ga0.52As second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type Al0.67Ga0.33As etching stop layer by the conventional photolithography and selective etching, and an n-type Al0.7Ga0.3As and n-type GaAs materials are embedded in both sides of the ridge structure by selective MOCVD using Cl2 gas. Then, the insulation film is removed, and thereafter a p-type GaAs layer is formed. Thus, a current confinement structure and an index-guided structure are built in the semiconductor laser device.
However, the above semiconductor laser device has a drawback that it is very difficult to form the AlGaAs second upper cladding layer on the AlGaAs first upper cladding layer, since the AlGaAs first upper cladding layer contains a high Al content and is prone to oxidation, and selective growth of the AlGaAs second upper cladding layer is difficult.
(2) In addition, IEEE Journal of Selected Topics in Quantum Electronics, vol. 29, No. 6, 1993, pp.1936 discloses a semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 to 1.02 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al0.4Ga0.6As lower cladding layer, an undoped Al0.2Ga0.8As optical waveguide layer, a GaAs/InGaAs double quantum well active layer, an undoped Al0.2Ga0.8As optical waveguide layer, a p-type Al0.4Ga0.6As upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above a mid-thickness of the p-type Al0.4Ga0.6As upper cladding layer by the conventional photolithography and selective etching, and an n-type In0.5Ga0.5P material and an n-type GaAs material are embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and then electrodes are formed. Thus, a current confinement structure and an index-guided structure are realized in the layered construction.
However, the above semiconductor laser device also has a drawback that it is very difficult to form the InGaP layer on the AlGaAs upper cladding layer, since the AlGaAs upper cladding layer contains a high Al content and is prone to oxidation, and it is difficult to grow an InGaP layer having a different V-group component, on such an upper cladding layer.
(3) Further, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.189 discloses an all-layer-Al-free semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type InGaP cladding layer, an undoped InGaAsP optical waveguide layer, an InGaAsP tensile strain barrier layer, an InGaAs double quantum well active layer, an InGaAsP tensile strain barrier layer, an undoped InGaAsP optical waveguide layer, a p-type InGaP first upper cladding layer, a p-type GaAs optical waveguide layer, a p-type InGaP second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type InGaP first upper cladding layer by the conventional photolithography and selective etching, and an n-type In0.5Ga0.5P material is embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and a p-type GaAs contact layer is formed. Thus, a current confinement structure and an index-guided structure are realized.
The reliability of the above semiconductor laser device is improved since the strain in the active layer can be compensated for. However, the above semiconductor laser device also has a drawback that the kink level is low (about 150 mW) due to poor controllability of the ridge width.
As in the above example, the conventional current semiconductor laser devices which contain a current confinement structure, oscillate in a fundamental transverse mode, and emit light in the 0.9 to 1.1 xcexcm band, are difficult to produce, or have poor characteristics, and are unreliable in high output power operation.
(4) High-power semiconductor laser devices having a broad light-emitting area are employed as excitation light sources in conventional solid-state laser apparatuses, in which output laser light is emitted from a solid-state laser crystal. In particular, some solid-state laser apparatuses further include a nonlinear crystal which converts a fundamental wave emitted from the solid-state laser crystal into a second harmonic, and such solid-state laser apparatuses are widely used.
In the above solid-state laser apparatuses, the semiconductor laser devices as excitation light sources are required to emit laser light with very high output power. In order to achieve the high output power, semiconductor laser devices in which an active layer has a width of 10 xcexcm or greater are used, while the widths of the active layers in single-mode laser devices are usually about 3 xcexcm. Therefore, a number of high-order transverse modes are mixed in oscillated light, and when the oscillation power is increased, the modes of oscillated light are liable to change to different modes due to spatial hole burning of carriers, which is caused by high density distribution of photons in the resonant cavity. At the same time, near-field pattern, far-field pattern, and oscillation spectrum vary. In addition, since efficiencies of current-to-light conversion are different between different transverse modes, the optical output power further varies. This phenomenon is called a kink in the current-optical output power characteristic of a semiconductor laser device.
Thus, the following problems arise.
When the above high-power semiconductor laser device is used as an excitation light source in a solid-state laser apparatus, at least one component coupled with an oscillation mode of the solid-state laser resonator is utilized as an excitation light from among oscillated light generated by the semiconductor laser device and condensed by a lens system into a solid-state laser crystal. Therefore, the output intensity varies greatly with changes of the transverse modes. In addition, since the absorption spectrum of the solid-state laser crystal has a fine absorption spectrum structure in a narrow wavelength band, an amount of absorbed light varies in response to the variation of an oscillation spectrum. Thus, the output intensity of the solid-state laser apparatus is further affected by variation of the oscillation spectrum, as well as change of the transverse modes. Furthermore, use of a spatial or spectral portion of the light generated by the solid-state laser device increases high-frequency noise accompanying by switching between the transverse modes.
As mentioned above, when transverse modes or longitudinal modes in a semiconductor laser device used as an excitation light source in a solid-state laser apparatus change, i.e., when an oscillation spectrum of the semiconductor laser device varies, the excitation efficiency in the solid-state laser apparatus varies, and therefore the optical output also varies. At the same time, high frequency noise is generated. Further, in practice, in order to vary the light intensity of the solid-state laser apparatus, or achieve phase matching with a wavelength conversion element, temperature and excitation current must be varied. Therefore, when oscillation modes change at the same time as the variations of the temperature and the excitation current, the intensity of the output laser light varies strikingly. Although the above variation of the intensity of the output laser light is a variation in the intensity of a DC component of the output laser light, it is probable that AC noise is continuously produced.
Generally, the intensity and the frequency spectrum of the output laser light of the solid-state laser apparatus depend on the intensity and the spectrum component of the excitation laser light which is emitted from the semiconductor laser device, and actually used for excitation of the solid-state laser crystal. In addition, the intensity and the frequency spectrum of the output laser light of the solid-state laser apparatus also vary with the excitation current and an individual difference in the characteristics of the semiconductor laser device. Thus, the intensity and the frequency spectrum of the output laser light of the solid-state laser apparatus are not uniform. The variation in the intensity of the DC laser light sometimes exceeds 10%, and therefore causes problems in various applications. In particular, in order to generate a high quality image, it is desirable that a noise level is 1% or lower. However, it is impossible to repetitively achieve or maintain the noise level of 1% or lower in the conventional high-power semiconductor laser devices which have an ordinary oscillation region. In addition, when a solid-state laser apparatus and a nonlinear crystal are combined in order to generate a second harmonic, the above noise is amplified by the nonlinear effect, and it is therefore necessary to further suppress the noise.
Japanese Unexamined Patent Publication, No.11(1999)-74620, which is assigned to the present assignee, discloses that reduction of a strain imposed on the semiconductor laser device by a so-called junction-up type mounting of the semiconductor laser device on a heat sink, as well as prevention of change of the transverse modes by using an index-guided semiconductor laser device, is effective at suppressing noise. However, it is difficult to increase an output power of a semiconductor laser device which is mounted on a heat sink in a junction-up configuration. In addition, photon density in an active layer must be suppressed in order to increase reliability, and the width of the optical waveguide layers must be made broad in order to suppress the photon density in an active layer. However, it is impossible to form an index-guided structure when the optical waveguide layers are broad.
As mentioned before, the conventional high-power semiconductor laser devices having a broad light-emitting region lack optical stability. That is, the optical output of the conventional high-power semiconductor laser devices having a broad light-emitting region is unstable, and the noise level in the optical output is not sufficiently low. Therefore, it is not desirable to use solid-state laser apparatuses using the conventional high-power semiconductor laser device as an excitation light source, as well as optical fiber laser apparatuses in which the conventional high-power semiconductor laser device is coupled to an optical fiber, in the applications for producing high quality images such as printed images, photographs, and medical images.
An object of the present invention is to provide a reliable semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high.
Another object of the present invention is to provide a process for producing a reliable semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high.
Still another object of the present invention is to provide a stripe-type index-guided semiconductor laser device which oscillates in multiple transverse modes, and has a stable optical output with low noise.
A further object of the present invention is to provide a process for producing a stripe-type index-guided semiconductor laser device which oscillates in multiple transverse modes, and has a stable optical output with low noise.
(1) According to the first aspect of the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer formed on the lower cladding layer; a compressive strain quantum well active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 and formed on the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4, 0xe2x89xa6y3xe2x89xa60.1, and an absolute value of a first product of a strain and a thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; an upper optical waveguide layer formed on the compressive strain quantum well active layer; a first upper cladding layer made of Inx8Ga1xe2x88x92x8P of a second conductive type, and formed on the upper optical waveguide layer; an etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 of the second conductive type, and formed on the first upper cladding layer other than a stripe area of the first upper cladding layer so as to form a first portion of a stripe groove realizing a current injection window, where 0xe2x89xa6x1xe2x89xa60.3, 0y1xe2x89xa60.3, and an absolute value of a second product of a strain and a thickness of the etching stop layer is equal to or smaller than 0.25 nm; a current confinement layer made of Inx8Ga1xe2x88x92x8P of the first conductive type, and formed on the etching stop layer so as to form a second portion of the stripe groove, where x8=0.49xc2x10.01; a second upper cladding layer made of Alz4Ga1xe2x88x92z4As of the second conductive type, and formed over the current confinement layer and the stripe area of the first upper cladding layer so as to cover the stripe groove, where 0.20xe2x89xa6z4xe2x89xa60.50; and a contact layer of the second conductive type, formed on the second upper cladding layer. In the semiconductor laser device, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate.
The first and second conductive types are opposite to each other in carrier polarity. For example, when the first conductive type is n type, the second conductive type is p type. In addition, the undoped type semiconductor is doped with substantially no impurity.
The strain xcex94a of the quantum well active layer is defined as xcex94a=(caxe2x88x92cs)/cs, where cs and ca are the lattice constants of the GaAs substrate and the quantum well active layer, respectively, and the strain xcex94e of the etching stop layer is defined as xcex94e=(cexe2x88x92cs)/cs, where ce is the lattice constant of the etching stop layer. That is, in the semiconductor laser device according to the first aspect of the present invention, xe2x88x920.25 nmxe2x89xa6xcex94axc2x7daxe2x89xa60.25 nm, and xe2x88x920.25 nmxe2x89xa6xcex94exc2x7dexe2x89xa60.25 nm, where da and de are the thicknesses of the quantum well active layer and the etching stop layer, respectively. In the first to fourth aspects of the present invention, when a layer grown over the substrate has a lattice constant c, and an absolute value of the amount (cxe2x88x92cs)/cs is equal to or smaller than 0.003, the layer is lattice-matched with the substrate.
Preferably, the semiconductor laser device according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii).
(i) The semiconductor laser device may further include first and second tensile strain barrier layers both made of Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5, and respectively formed above and below the compressive strain quantum well active layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0xe2x89xa6y5xe2x89xa60.6, and an absolute value of a sum of the first product and a third product of a strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.
The strain xcex94b of the first and second tensile strain barrier layers is defined as xcex94b=(cbxe2x88x92cs)/cs, where cb is the lattice constant of the first and second tensile strain barrier layers, and cs is the lattice constant of the substrate. That is, in this semiconductor laser device, xe2x88x920.25 nmxe2x89xa6xcex94axc2x7da+xcex94bxc2x7dbxe2x89xa60.25 nm, where db is the total thickness of the first and second tensile strain barrier layers.
(ii) The semiconductor laser device may further include an additional layer being made of Inx8Ga1xe2x88x92x8P of the second conductive type, formed below the second upper cladding layer, and having a thickness between 10 to 400 nm, where x8=0.49xc2x10.01. In particular, the optimum thickness of the Inx8Ga1xe2x88x92x8P layer is 250 to 300 nm.
(iii) The stripe groove may have a width equal to or greater than 1 xcexcm.
(2) According to the second aspect of the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer formed on the lower cladding layer; a compressive strain quantum well active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3, and formed on the lower optical waveguide layer, where 0xe2x89xa6x3xe2x89xa60.4, and 0xe2x89xa6y3xe2x89xa60.1, and an absolute value of a first product of a strain and a thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; an upper optical waveguide layer formed on the compressive strain quantum well active layer; a first upper cladding layer of a second conductive type, formed on the upper optical waveguide layer; a first etching stop layer made of Inx7Ga1xe2x88x92x7P of the second conductive type, and formed on the first upper cladding layer, where 0xe2x89xa6x7xe2x89xa61; a second etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 of the second conductive type, and formed on the first etching stop layer other than a stripe area of the first etching stop layer so as to form a first portion of a stripe groove realizing a current injection window, where 0xe2x89xa6x1xe2x89xa60.3, and 0xe2x89xa6y1xe2x89xa60.3; a current confinement layer made of Inx8Ga1xe2x88x92x8P of the first conductive type, and formed on the second etching stop layer so as to form a second portion of the stripe groove, where x8=0.49xc2x10.01; a second upper cladding layer made of Alz4Ga1xe2x88x92z4As of the second conductive type, and formed over the current confinement layer and the stripe area of the first upper cladding layer so as to cover the stripe groove, where 0.20xe2x89xa6z4xe2x89xa60.50; and a contact layer of the second conductive type, formed on the second upper cladding layer. In the semiconductor laser device, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate, and an absolute value of a sum of a second product of a strain and a thickness of the first etching stop layer and a third product of a strain and a thickness of the second etching stop layer is equal to or smaller than 0.25 nm. That is, in the semiconductor laser device according to the second aspect of the present invention, xe2x88x920.25 nmxe2x89xa6xcex94e1xc2x7de1+xcex94e2xc2x7de2xe2x89xa60.25 nm, where xcex94e1 and xcex94e2 are strains of the first and second etching stop layers, respectively, and de1 and de2 are the thicknesses of the first and second etching stop layers, respectively.
Preferably, the semiconductor laser device according to the second aspect of the present invention may also have one or any possible combination of the aforementioned additional features (ii) and (iii) and the following additional features (iv) and (v).
(iv) The semiconductor laser device may further include first and second tensile strain barrier layers both made of Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5, and respectively formed above and below the compressive strain quantum well active layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6, and an absolute value of a sum of the first product and a fourth product of a strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.
(v) The first upper cladding layer is made of Inx6Ga1xe2x88x92x6As1xe2x88x92y6P6y or Alz5Ga1xe2x88x92z5As, where x6=(0.49xc2x10.01)y6, 0.2 less than y6 less than 1, and 0.25xe2x89xa6z5xe2x89xa60.7.
(3) According to the third aspect of the present invention, there is provided a process for producing a semiconductor laser device, comprising the steps of: (a) forming a lower cladding layer of a first conductive type, on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer on the lower cladding layer; (c) forming a compressive strain quantum well active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3, on the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4, 0xe2x89xa6y3xe2x89xa60.1, and an absolute value of a first product of a strain and a thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; (d) forming an upper optical waveguide layer on the compressive strain quantum well active layer; (e) forming a first upper cladding layer made of Inx8Ga1xe2x88x92x8P of a second conductive type, on the upper optical waveguide layer; (f) forming an etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 of the second conductive type, on the first upper cladding layer, where 0xe2x89xa6x1xe2x89xa60.3, 0xe2x89xa6y1xe2x89xa60.3, and an absolute value of a second product of a strain and a thickness of the etching stop layer is equal to or smaller than 0.25 nm; (g) forming a current confinement layer made of Inx8Ga1xe2x88x92x8P of the first conductive type, on the etching stop layer, where x8=0.49xc2x10.01; (h) removing a stripe area of the current confinement layer so as to form a first portion of a stripe groove for realizing a current injection window; (i) removing a stripe area of the etching stop layer so as to form a second portion of the stripe groove; (j) forming a second upper cladding layer made of Alz4Ga1xe2x88x92z4As of the second conductive type so that the stripe groove is covered with the second upper cladding layer, where 0.20xe2x89xa6z4xe2x89xa60.50; and (k) forming a contact layer of the second conductive type, on the second upper cladding layer. In the process, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate.
That is, the semiconductor laser device according to the first aspect of the present invention can be produced by the process according to the third aspect of the present invention.
Preferably, the process according to the third aspect of the present invention may also have one or any possible combination of the following additional features (vi) to (viii).
(vi) The process according to the third aspect of the present invention may further comprise the steps of (b1) forming, after the step (b), a first tensile strain barrier layer made of Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5, on the lower optical waveguide layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6, and (c1) forming, after the step (c), a second tensile strain barrier layer made of Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5, on the compressive strain quantum well active layer, where an absolute value of a sum of the first product and a third product of a strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.
(vii) The process according to the third aspect of the present invention may further comprise the step of (j1) forming, before the step (j), an additional layer having a thickness of 10 to 400 nm and being made of Inx8Ga1xe2x88x92x8P of the second conductive type on the current confinement layer so that the stripe groove is covered with the additional layer, where x8=0.49xc2x10.01. In particular, it is preferable that the thickness of the Inx8Ga1xe2x88x92x8P layer is 250 to 300 nm.
(viii) The process according to the third aspect of the present invention may further comprise, after the step (g), the steps of (g1) forming a cap layer made of GaAs, and (g2) removing a stripe area of the cap layer. In addition, in the step (i), a remaining area of the cap layer is also removed.
The above cap layer may be one of the first and second conductive types and the undoped type.
(4) According to the fourth aspect of the present invention, there is provided a process for producing a semiconductor laser device, comprising the steps of: (a) forming a lower cladding layer of a first conductive type on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer on the lower cladding layer; (c) forming a compressive strain quantum well active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 on the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4, and 0xe2x89xa6y3xe2x89xa60.1, and an absolute value of a first product of a strain and a thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; (d) forming an upper optical waveguide layer on the compressive strain quantum well active layer; (e) forming a first upper cladding layer of a second conductive type, on the upper optical waveguide layer; (f) forming a first etching stop layer made of Inx7Ga1xe2x88x92x7P of the second conductive type, on the first upper cladding layer, where 0xe2x89xa6x7xe2x89xa61; (g) forming a second etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 of the second conductive type, on the first etching stop layer, where 0xe2x89xa6x1xe2x89xa60.3, and 0xe2x89xa6y1xe2x89xa60.3; (h) forming a current confinement layer made of Inx8Ga1xe2x88x92x8P of the first conductive type, on the second etching stop layer, where x8=0.49xc2x10.01; (i) removing a stripe area of the current confinement layer so as to form a first portion of a stripe groove for realizing a current injection window; (j) removing a stripe area of the second etching stop layer so as to form a second portion of the stripe groove; (k) forming a second upper cladding layer made of Alz4Ga1xe2x88x92z4As of the second conductive type so that the stripe groove is covered with the second upper cladding layer, where 0.20xe2x89xa6z4xe2x89xa60.50; and (1) forming a contact layer of the second conductive type, on the second upper cladding layer. In the process, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate, and an absolute value of a sum of a second product of a strain and a thickness of the first etching stop layer and a third product of a strain and a thickness of the second etching stop layer is equal to or smaller than 0.25 nm.
That is, the semiconductor laser device according to the second aspect of the present invention can be produced by the process according to the fourth aspect of the present invention.
Preferably, the process according to the fourth aspect of the present invention may also have one or any possible combination of the aforementioned additional feature (vi) and the following additional features (ix) and (x).
(ix) The process according to the fourth aspect of the present invention may further comprise the step of (k1) forming, before the step (k), an additional layer having a thickness of 10 to 400 nm and being made of Inx8Ga1xe2x88x92x8P of the second conductive type on the current confinement layer so that the stripe groove is covered with the additional layer, where x8=0.49xc2x10.01. In particular, it is preferable that the thickness of the Inx8Ga1xe2x88x92x8P layer is 250 to 300 nm.
(x) The process according to the fourth aspect of the present invention may further comprise the steps of (h1) forming a cap layer made of GaAs on the current confinement layer, and (h2) removing a stripe area of the cap layer. In addition, in the step (j), a remaining area of the cap layer is also removed.
The above cap layer may be one of the first and second conductive types and the undoped type.
(5) The first to fourth aspects of the present invention have the following advantages.
(a) In the semiconductor laser devices according to the first and second aspects of the present invention, the current confinement layer is made of Inx8Ga1xe2x88x92x8P, and the second upper cladding layer is made of Alz4Ga1xe2x88x92z4As. Therefore, the difference in the refractive indexes between the current confinement layer and the second upper cladding layer realizes, with high accuracy, a difference of about 1.5xc3x9710xe2x88x923 to 1xc3x9710xe2x88x922 in the equivalent refractive index between a portion of the active region under the stripe groove and the other portions of the active region under the current confinement layer, and it is possible to cut off oscillation in higher modes. Thus, oscillation in a fundamental transverse mode can be maintained even when the output power becomes high.
(b) When a base layer on which the second upper cladding layer is formed contains aluminum, the base layer is prone to oxidation, and it is difficult to realize desired characteristics in the semiconductor laser device. However, in the semiconductor laser devices according to the first and second aspects of the present invention, the Inx8Ga1xe2x88x92x8P first upper cladding layer, the Inx7Ga1xe2x88x92x7P first etching stop layer, and the Inx8Ga1xe2x88x92x8P current confinement layer, which can be a base layer of the second upper cladding layer, do not contain aluminum. Therefore, it is easy to form the second upper cladding layer. In addition, since a crystal defect due to oxidation of aluminum does not occur, the characteristics of the semiconductor laser device do not deteriorate, and reliability is improved.
(c) Since the current confinement layer is arranged within the semiconductor laser device, it is possible to increase the contact area between the electrode and the contact layer. Therefore, the contact resistance can be reduced.
(d) Due to the provision of the current confinement layer, the currents can be confined within a small width during current injection into the active region. Therefore, the transverse mode oscillation is less prone to cause a kink in a current-optical output characteristic. That is, the kink level can be raised.
(e) Since the etching stop layer is made of InGaAsP, the stripe width can be accurately adjusted by wet etching in which the difference in the etching rate between the etching stop layer and the InGaP current confinement layer is utilized.
(f) When the InGaP layer having a thickness of 10 to 400 nm is formed before the second upper cladding layer is formed, it is possible to increase the control range of the A1 composition in the Alz4Ga1xe2x88x92z4As second upper cladding layer.
(g) When the Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5 tensile strain barrier layers (0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6) are respectively formed above and below the compressive strain quantum well active layer, various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased.
(h) When the stripe width is equal to or greater than 1 xcexcm, the semiconductor laser devices according to the first and second aspects of present invention are most advantageous since the semiconductor laser devices can oscillate with high output power and low noise, even in multiple modes.
(i) When a GaAs cap layer is formed on the InGaP current confinement layer, it is possible to prevent formation of a natural oxidation film on the InGaP current confinement layer, as well as metamorphic change in the InGaP current confinement layer, which occurs when a resist layer is formed directly on the InGaP current confinement layer. In addition, since the GaAs cap layer is removed before the second upper optical waveguide layer is formed, it is possible to remove a residue left on the base layer on which the second upper optical waveguide layer is formed, and prevent the occurrence of crystal defects.
(6) According to the fifth aspect of the present invention, there is provided a semiconductor laser device comprising: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer of an undoped type or the first conductive type, formed on the lower cladding layer; an active layer formed on the lower optical waveguide layer; a first upper optical waveguide layer of an undoped type or a second conductive type, formed on the active layer; an etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 of the second conductive type, and formed on the first upper optical waveguide layer other than a stripe area of the first upper optical waveguide layer so as to form a first portion of a stripe groove realizing a current injection window, where 0xe2x89xa6x1xe2x89xa60.5, and 0xe2x89xa6y1xe2x89xa60.8; a current confinement layer made of Inx3(Alz3Ga1xe2x88x92z3)1xe2x88x92x3P of the first conductive type, and formed on the etching stop layer so as to form a second portion of the stripe groove, where 0 less than z3xe2x89xa61, and x3=0.49xc2x10.01; a second upper optical waveguide layer of the second conductive type, formed over the current confinement layer and the stripe area of the first upper optical waveguide layer so as to cover the stripe groove; an upper cladding layer of the second conductive type, and formed on the second upper optical waveguide layer; and a contact layer made of GaAs of the second conductive type, and formed on the upper cladding layer. In the semiconductor laser device, a total thickness of the lower optical waveguide layer and the first and second upper optical waveguide layers is equal to or greater than 0.6 xcexcm, and the active layer is made of one of InGaAs, InGaAsP, and GaAsP.
Preferably, the semiconductor laser device according to the fifth aspect of the present invention may also have one or any possible combination of the following additional features (xi) and (xvii).
(xi) The process according to the fifth aspect of the present invention may further comprise a cap layer made of In0.49Ga0.51P of the first or second conductive type, and formed between the current confinement layer and the second upper optical waveguide layer.
(xii) Each of the lower optical waveguide layer and the first and second upper optical waveguide layers is made of Inx2Ga1xe2x88x92x2p, where x2=0.49xc2x10.01.
(xiii) Each of the lower optical waveguide layer and the first and second upper optical waveguide layers is made of Inx2Ga1xe2x88x92x2Asy2P1xe2x88x92y2, where x2=(0.49xc2x10.01)y2, and 0xe2x89xa6x2xe2x89xa60.49.
(xiv) The semiconductor laser device according to the fifth or sixth aspect of the present invention further comprises first and second tensile strain barrier layers made of one of InGaP, InGaAsP, and GaAsP, and respectively formed above and below the active layer.
(xv) Each of the lower and upper cladding layers is made of one of AlGaAs, InGaAlP, and InGaAlPAs which lattice-match with the GaAs substrate.
In the fifth to tenth aspects of the present invention, when a layer grown over the substrate has a lattice constant c, and an absolute value of the amount (cxe2x88x92cs)/cs is equal to or smaller than 0.003, the layer is lattice-matched with the substrate, where cs is the lattice constant of the GaAs substrate.
(xvi) The bottom of the stripe groove has a width of 1 to 5 xcexcm, and a difference in an equivalent refractive index caused by a difference in a refractive index between the current confinement layer and the second upper optical waveguide layer is in a range from 0.0015 to 0.01.
The difference in the equivalent refractive index is a difference in the equivalent refractive index in propagation modes in the thickness direction, between portions of the active region under the current confinement layer and the other portion of the active region under the stripe groove.
(xvii) The bottom of the stripe groove has a width equal to or greater than 10 xcexcm.
(7) According to the sixth aspect of the present invention, there is provided a semiconductor laser device comprising: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer of an undoped type or the first conductive type, formed on the lower cladding layer; an active layer formed on the lower optical waveguide layer; a first upper optical waveguide layer of an undoped type or a second conductive type, formed on the active layer; a first etching stop layer made of Inx9Ga1xe2x88x92x9P of the second conductive type, and formed on the first upper optical waveguide layer, where 0xe2x89xa6x9xe2x89xa61; a second etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 of the second conductive type, and formed on the first etching stop layer other than a stripe area of the first etching stop layer so as to form a first portion of a stripe groove realizing a current injection window, where 0xe2x89xa6x1xe2x89xa60.5, and 0xe2x89xa6y1xe2x89xa60.8; a current confinement layer made of Inx3(Alz3Ga1xe2x88x92z3)1xe2x88x92x3P of the first conductive type, and formed on the second etching stop layer so as to form a second portion of the stripe groove, where 0 less than z3xe2x89xa61, and x3=0.49xc2x10.01; a second upper optical waveguide layer of the second conductive type, formed over the current confinement layer and the stripe area of the first etching stop layer so as to cover the stripe groove; an upper cladding layer of the second conductive type, and formed on the second upper optical waveguide layer; and a contact layer made of GaAs of the second conductive type, and formed on the upper cladding layer. In the semiconductor laser device, a total thickness of the lower optical waveguide layer and the first and second upper optical waveguide layers is equal to or greater than 0.6 xcexcm, and the active layer is made of one of InGaAs, InGaAsP, and GaAsP.
Preferably, the semiconductor laser device according to the sixth aspect of the present invention may also have one or any possible combination of the aforementioned additional features (xi) and (xvii).
(8) According to the seventh aspect of the present invention, there is provided a process for producing a semiconductor laser device, comprising the steps of: (a) forming a lower cladding layer of a first conductive type, on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer of an undoped type or the first conductive type, on the lower cladding layer; (c) forming an active layer on the lower optical waveguide layer; (d) forming a first upper optical waveguide layer of an undoped type or a second conductive type, on the active layer; (e) forming an etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 of the second conductive type, on the first upper optical waveguide layer, where 0xe2x89xa6x1xe2x89xa60.5, and 0xe2x89xa6y1xe2x89xa60.8; (f) forming a current confinement layer made of Inx3(Alz3Ga1xe2x88x92z3)1xe2x88x92x3P of the first conductive type, on the etching stop layer, where 0 less than z3xe2x89xa61, and x3=0.49xc2x10.01, (g) removing a stripe area of the current confinement layer so as to form a first portion of a stripe groove for realizing a current injection window; (h) removing a stripe area of the etching stop layer so as to form a second portion of the stripe groove; (i) forming a second upper optical waveguide layer of the second conductive type so that the stripe groove is covered with the second upper optical waveguide layer; (j) forming an upper cladding layer of the second conductive type, on the second upper optical waveguide layer; and (k) forming a contact layer made of GaAs of the second conductive type, on the upper cladding layer. In the process, a total thickness of the lower optical waveguide layer and the first and second upper optical waveguide layers is equal to or greater than 0.6 xcexcm, and the active layer is made of one of InGaAs, InGaAsP, and GaAsP.
That is, the semiconductor laser device according to the fifth aspect of the present invention can be produced by the process according to the seventh aspect of the present invention.
Preferably, the process according to the seventh aspect of the present invention may also have one or any possible combination of the following features (xviii) and (xix).
(xviii) The process according to the seventh aspect of the present invention may further comprise the steps of (b1) forming, after the step (b), a first tensile strain barrier layer made of one of InGaP, InGaAsP, and GaAsP, on the lower optical waveguide layer, and (c1) forming, after the step (c), a second tensile strain barrier layer made of one of InGaP, InGaAsP, and GaAsP, on the active layer.
(xix) The process according to the seventh aspect of the present invention may further comprise, after the step (f), the steps of (f1) forming a cap layer made of In0.49Ga0.51P of the first or second conductive type, and (f2) removing a stripe area of the cap layer. In addition, in the step (h), a remaining area of the cap layer is also removed.
(9) According to the eighth aspect of the present invention, there is provided a process for producing a semiconductor laser device, comprising the steps of: (a) forming a lower cladding layer of a first conductive type, on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer of an undoped type or the first conductive type, on the lower cladding layer; (c) forming an active layer on the lower optical waveguide layer; (d) forming a first upper optical waveguide layer of an undoped type or a second conductive type, on the active layer; (e) forming a first etching stop layer made of Inx9Ga1xe2x88x92x9P of the second conductive type, on the first upper optical waveguide layer, where 0xe2x89xa6x9xe2x89xa61; (f) forming a second etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 of the second conductive type, on the first etching stop layer, where 0xe2x89xa6x1xe2x89xa60.5, and 0xe2x89xa6y1xe2x89xa60.8; (g) forming a current confinement layer made of Inx3(Alz3Ga1xe2x88x92z3)1xe2x88x92x3P of the first conductive type, on the second etching stop layer, where 0 less than z3xe2x89xa61, and x3=0.49xc2x10.01; (h) removing a stripe area of the current confinement layer so as to form a first portion of a stripe groove for realizing a current injection window; (i) removing a stripe area of the second etching stop layer so as to form a second portion of the stripe groove; (j) forming a second upper optical waveguide layer of the second conductive type so that the stripe groove is covered with the second upper optical waveguide layer; (k) forming an upper cladding layer of the second conductive type, on the second upper optical waveguide layer; and (1) forming a contact layer made of GaAs of the second conductive type, on the upper cladding layer. In the process, a total thickness of the lower optical waveguide layer and the first and second upper optical waveguide layers is equal to or greater than 0.6 xcexcm, and the active layer is made of one of InGaAs, InGaAsP, and GaAsP.
That is, the semiconductor laser device according to the sixth aspect of the present invention can be produced by the process according to the eighth aspect of the present invention.
Preferably, the process according to the eighth aspect of the present invention may also have one or any possible combination of the aforementioned feature (xviii) and the following feature (xx).
(xx) The process according to the eighth aspect of the present invention may further comprise, after the step (g), the steps of (g1) forming a cap layer made of In0.49Ga0.51P of the first or second conductive type, and (g2) removing a stripe area of the cap layer. In addition, in the step (i), a remaining area of the cap layer is also removed.
(10) According to the ninth aspect of the present invention, there is provided a solid-state laser apparatus having as an exciting light source the semiconductor laser device according to the fifth aspect of the present invention.
Preferably, the solid-state laser apparatus according to the ninth aspect of the present invention may also have one or any possible combination of the following additional feature (xxi) and the aforementioned additional features (xi) to (xvii).
(xxi) The solid-state laser apparatus according to the ninth aspect of the present invention may further comprise a solid-state laser crystal which is excited with first laser light emitted from the excitation light source, and emits second laser light, and a wavelength conversion crystal which converts the second laser light into a second harmonic.
(11) According to the tenth aspect of the present invention, there is provided a solid-state laser apparatus having as an exciting light source the semiconductor laser device according to the sixth aspect of the present invention.
Preferably, the solid-state laser apparatus according to the tenth aspect of the present invention may also have one or any possible combination of the aforementioned additional features (xi) to (xvii), and (xxi).
(12) The fifth to tenth aspects of the present invention have the following advantages.
(a) Since the current confinement structure is formed, the strain caused by the junction-down type mounting can be reduced, and the fluctuation in the transverse modes during oscillation can be suppressed. Thus, the variation in the optical output of the solid-state laser apparatus can be suppressed. In addition, when the junction-down type mounting is employed, the heat dissipation characteristics of the semiconductor laser device are improved. Therefore, the amount of variation in the wavelength, which is caused by increase in a driving current, can be reduced. Thus, it is possible to realize a solid-state laser apparatus which is highly reliable for a long time.
(b) In the semiconductor laser devices according to the fifth and sixth aspects of the present invention, the active layers do not contain aluminum. Therefore, the semiconductor laser devices are free from deterioration caused by oxidation of aluminum, and the semiconductor laser devices are reliable even when the semiconductor laser devices operate with high output power.
(c) In the semiconductor laser devices according to the fifth and sixth aspects of the present invention, the current confinement layer is made of Inx3(Alz3Ga1xe2x88x92z3)1xe2x88x92x3P. Therefore, when the second upper optical waveguide layer is made of InGaP or InGaAsP, the difference in the refractive indexes between the current confinement layer and the second upper cladding layer realizes a difference of about 1.5xc3x9710xe2x88x923 to 1xc3x9710xe2x88x922 in the equivalent refractive index between the portion of the active region under the stripe groove and the other portions of the active region under the current confinement layer. In particular, when the width of the oscillation region is narrow, e.g., 1 xcexcm to 5 xcexcm, and the difference in the equivalent refractive index is too great, the transverse modes become unstable. However, when the difference in the equivalent refractive index is about 1.5xc3x9710xe2x88x923 to 1xc3x9710xe2x88x922, the semiconductor laser devices can operate in a basic transverse mode even when the output power is high, and the instability of the transverse mode due to occurrence of higher modes can be prevented.
(d) Since the current confinement layer is arranged within the semiconductor laser device, it is possible to increase the contact area between the electrode and the contact layer. Therefore, the contact resistance can be reduced, and it is possible to operate the semiconductor laser devices with high output power.
(e) In order to prevent deterioration of a light-emitting end surface caused by high photon density in a high-power semiconductor laser device, it is effective to increase a thickness of an optical waveguide layer so as to reduce a peak photon density in an active layer. However, in conventional semiconductor laser devices having a current confinement structure and an index-guided structure, the thickness of the optical waveguide layer located between the active layer and the current confinement layer is limited since the distance between the active layer and the current confinement layer cannot be increased in order to realize a basic transverse mode by the effect of the index-guided structure. According to the fifth to tenth aspects of the present invention, the second upper optical waveguide layer is arranged over the current confinement layer, and the total thickness of the lower optical waveguide layer and the first and second upper optical waveguide layers is equal to or greater than 0.6 xcexcm. Therefore, it is possible to substantially increase the thickness of the optical waveguide layer, and reduce the peak photon density in the active layer. In addition, it is possible to prevent the temperature rise at a light-emitting end surface caused by increase in non-emission currents, and therefore deterioration of the light-emitting end surface due to high photon density can be prevented, thus increasing reliability in a high power operation.
(f) In the sixth, eighth, and tenth aspects of the present invention, the first etching stop layer is made of InGaP, and the second etching stop layer made of InGaAsP is arranged over the first etching stop layer. Therefore, when a sulfuric acid etchant is used, only the InGaAsP second etching stop layer is etched, and the InGaP first etching stop layer is not etched. That is, it is possible to stop the etching accurately on the upper boundary of the first etching stop layer, and thus the index-guided structure and the stripe width can be accurately formed by etching.
(g) When the InGaP cap layer is formed on the Inx3(Alz3Ga1xe2x88x92z3P current confinement layer, it is possible to prevent formation of a natural oxidation film on the Inx3(Alz3Ga1xe2x88x92z3)1xe2x88x92x3P current confinement layer, as well as metamorphic change in the Inx3(Alz3Ga1xe2x88x92z3)1xe2x88x92x3P current confinement layer, which occurs when a resist layer is formed directly on the Inx3(Alz3Ga1xe2x88x92z3)1xe2x88x92x3P current confinement layer.
(h) When each of the lower optical waveguide layer and the first and second upper optical waveguide layers is made of Inx2Ga1xe2x88x92x2p or Inx2Ga1xe2x88x92x2Asy2P1xe2x88x92y2, the band gap difference between the active layer and the optical waveguide layers can be made greater than the band gap differences in the conventional semiconductor laser devices. Therefore, the leakage current can be prevented, carriers can be efficiently confined, and thus the threshold current can be lowered.
(i) When the tensile strain barrier layers made of InGaP, InGaAsP, or GaAsP are formed above and below the compressive strain quantum well active layer, respectively, various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased.
(j) When each of the lower and upper cladding layers is made of one of AlGaAs, InGaAlP, and InGaAlPAs which lattice-match with the GaAs substrate, carriers and light can be effectively confined within the active layer, and the efficiency can be increased, since the band gaps of the cladding layers made of such materials are greater than the band gaps of the optical waveguide layers, and the refractive indexes of the cladding layers are smaller than the refractive indexes of the optical waveguide layers.
(k) Since the solid-state laser apparatuses according to the ninth and tenth aspects of the present invention use as excitation light sources the reliable, high power semiconductor laser devices according to the fifth and sixth aspects of the present invention, reliable, high power solid-state laser apparatuses are realized.
(1) In particular, when the semiconductor laser devices used in the solid-state laser apparatuses according to the ninth or tenth aspect of the present invention have an oscillation region (i.e., a stripe groove) with a width of 10 xcexcm or broader, reliable, high power laser light can be obtained.
(m) In particular, when the solid-state laser apparatuses according to the ninth or tenth aspect of the present invention comprise a wavelength conversion crystal which converts the solid-state laser light into a second harmonic, reliable, high power second harmonic laser light can be obtained.